In the double crucible method of making glass fibres two concentric bushings, typically of platinum, are furnished with molten core and cladding glasses, and fibre is drawn from the outfall of the nozzles at the bottom of the bushings. If the bushings are held at constant temperature and in fixed spatial relationship with each other, then the ratio of core to cladding diameters will be a function of core and cladding glass melt levels. If the bushings are not continuously replenished, then the levels change as the bushings drain, and in general this will cause the core to cladding diameter ratio to change. It is possible under certain circumstances to arrange the starting conditions so that the levels fall `in step` in such a way that the core to cladding diameter ratio remains constant. However, a significant disadvantage is that the fibre is produced in batches. Constant core to cladding ratio can in principle be obtained in a continuous process in which the melts are continuously topped up to maintain their levels constant. A difficulty with this approach is the measurement of the melt levels.